Why Black Lights Make Things Glow? The difference is that black lights emit most of their light waves just outside the range humans can perceive, in the Ultraviolet (UV) part of the spectrum. Black light bulbs have phosphor coating that absorbs the higher-end UV light. When a UV light wave hits an object containing substances known as phosphors, those phosphors will naturally fluoresce and glow. The bulb is coated with phosphorus. This phosphorus, when hit with photons, create heat as well as light. As a result of that particular combination of heat and light, the photons that the phosphors emit are just a little less active than the photons that the phosphorus absorb.